The temporal evolution of the negative ion density in a low duty cycle pulsed reactive magnetron discharge

Time-resolved O- density measurements have been made in the bulk plasma of a unipolar pulsed-DC magnetron using laser photodetachment. The magnetron was operated with a titanium target at a total pressure of 1.3 Pa and a fixed oxygen-to-argon partial pressure ratio of 10%. The duty cycle was maintained at 5% but the peak on-time discharge power was varied from 120 to 720 W. For all discharge powers, both the electron (n(e)) and negative ion (n_) densities increase during the plasma on-time, with the electron density reaching a maximum at the end of this phase. In the off-time, the electron density initially decreases at a rapid rate (characteristic decay time similar to 25 mu s) for the first 50 mu s, followed by a slower rate (similar to 150 mu s) for the remainder of the off-time, however, the negative ion density continues to increase in this phase, reaching a maximum at about similar to 150 mu s after the termination of the discharge power. Both the electron and negative ion densities increase with discharge power. The maximum negative ion density was 2.5 x 10(16) m(-3) for a peak power of 720 W, corresponding to a negative ion-to-electron density ratio n_/n(e) (alpha) of about 3. In the long afterglow, this ratio reaches a maximum value of 12 as the electron density decreases faster than the negative ion density. This shows that in the afterglow the plasma is highly electronegative. (C) 2011 Elsevier B.V. All rights reserved.